A conventional seat belt system comprises a length of seat belt webbing connected at three points to load-bearing parts of a vehicle. A lap portion of the seat belt webbing passes laterally across the hips of the seat occupant, and a torso portion of the seat belt webbing passes diagonally across the torso of the seat occupant from one hip to the opposite shoulder.
Typically one end of the seat belt webbing is attached to a sill anchor that is bolted to a load-bearing part of the vehicle on one side of the seat, usually between the seat and an adjacent door. The lap and torso portions of the seat belt webbing join at a buckle mechanism on the opposite side of the seat. The shoulder end of the seat belt webbing is attached to a seat belt retractor mounted to a load-bearing part of the vehicle, for example a side pillar or sill, or directly to a load-bearing seat, optionally via a webbing guide.
The seat belt retractor increases comfort for the seat occupant restrained by the seat belt since it allows the seat belt webbing to pay out under relatively low loads to enable limited movement of the restrained seat occupant, for example to reach entertainment controls or storage compartments. The seat belt retractor is biased to keep the seat belt webbing relatively taut about the seat occupant. A locking element locks the seat belt retractor against webbing payout in the event an acceleration sensor senses a crash.
The seat belt webbing is fastened to the buckle mechanism by a buckle tongue that is slidably attached to the seat belt webbing so that the lengths of seat belt webbing making up the lap and torso portions can easily be varied to suit the size and shape of the vehicle occupant.
Known seat belt systems of this sort tend to be unsuitable for vehicle occupants of shorter than average stature, particularly for children, because the shoulder fastening point is fixed to accommodate an average person and is fixed at or above the height of the back of the seat. This is particularly so in rear seat safety restraints. Thus the torso portion of the seat belt tends to be badly positioned for a child or short person and usually passes too close or adjacent to the child's neck. Because the child does not fit into the adult seat belt properly the child's shoulder can roll out of the seat belt during a crash effectively making the seat belt a two-point lap belt only. This is dangerous because the lap portion alone will then take more force in a crash and will be more likely to inflict injuries than when a torso belt portion is combined with the lap portion. In addition there is a danger of the child sliding under the lap portion; this is known as submarining. It is well known that children feel uncomfortable with adult seat belt restraints and often position the torso portion behind their back to reduce discomfort.